Advanced fiber reinforced, organic matrix, composite materials provide designers of high performance structures with significant advantages in strength-to-weight and stiffness-to-weight as well as resistance to environmental corrosion when compared to metals. Also, in several applications, including load bearing medical implants, the ability to adjust the modulus of elasticity of a composite by varying fiber orientation, concentration and type is used to great advantage in achieving optimal load distribution in comparison with metallic structures characterized by a single, uniform modulus of elasticity. Such metallic structures are characterized by high, fixed moduli of elasticity which makes it difficult to achieve optimal device stiffness within a given anatomical geometric envelope.
Both thermoset and thermoplastic polymers have found application as the matrix in these composite systems. Thermoplastic composites have received increasing attention in recent years because of their greater fracture toughness when compared to thermoset systems. In addition, thermoplastics offer the advantages of thermal processing instead of the reactive cure processing characteristic of thermosets.
However, fabrication of thermoplastic matrix composite structures, especially those of complex shape, offers considerable challenge. Although flow of the fully polymerized matrix resin occurs with the application of heat and pressure, thermoplastic melts are usually characterized by high viscosities often several orders of magnitude higher than uncured thermosets. Impregnation of the reinforcement fiber bundles is, therefore, difficult and consolidation of the resultant structure to eliminate residual voids, which can be deleterious to mechanical performance, is often incomplete. Well known impregnation and consolidation processes effectively used with thermosets, including resin transfer molding, are usually ineffective with thermoplastics.
One process for fabricating thermoplastic matrix composite systems which is capable of high consolidation is compression molding. Lamina of preimpregnated composite material are layed up at the desired fiber orientation and placed in a closed mold and subjected to heat and pressure to produce a well consolidated mass. Alternatively, alternating layers of fiber and matrix film or comingled bundles of reinforcing fiber and matrix fiber may be used in place of the preimpregnated lamina. In anycase, this process is largely limited to the production of structures of simple shape; specifically, flat or simply curved shapes. Besides the limitation in shape, these parts are often characterized by the appearance of cut fiber ends on four sides which can lead to delamination in service. The formation of a complex shape, such as an orthopedic implant, from such a compression molded mass requires additional machining operations which lead to further exposure of additional free fiber ends.
U.S. Pat. No. 4,892,552 describes production of a composite orthopedic device by machining a composite block itself fabricated by compression molding. European Patent Publication 0 277 727 also describes production of a composite orthopedic device from a block fabricated by compression molding. Prostheses of this reference are formed from plies of continuous filament fibers which are stacked and compression molded. In the latter case the molded block is simply curved but fiber ends are still exposed at the edges. Furthermore, the finished devices are obtained only by additional machining and the like. Articles produced according to the present invention may be formed as finished parts within the mold itself.
More preferred approaches to the production of composite structures with significant curvature and/or complex shape include nearer net shape fiber placement processes such as filament winding and braiding. These processes are capable of producing parts comprising closed contours, thus, minimizing free fiber ends and potential sites of delamination. In addition, these processes are compatible with the incorporation of varying fiber orientation within a structure so that local stiffness and strength can be tailored to the mechanical requirements of the application. Furthermore, these processes are easily mechanized and computerized making them suitable for production at high efficiency and low cost.
In situ consolidation of thermoplastic composites has been demonstrated during filament winding and braiding by application of heat and fiber tension during the placement of reinforcing tow preimpregnated with matrix. See for example, M. W. Egerton and M. B. Gruber, "Thermoplastic Filament Winding Demonstrating Economies and Properties Via In-Situ Consolidation", Proceedings of the 33rd International SAMPE Symposium, 1988, pp. 35-46. Nevertheless, experience with such processes suggests that further consolidation is often desirable. Even if complete fusion occurs at the interface between the composite tow and the previously wound or braided mass, voids may still be present in the structure as a result of incomplete consolidation in the earlier tow manufacturing process. In addition, voids may remain in the as-wound or as-braided part as a result of upsets in the winding or braiding process including tow breaks. Further consolidation may also be desirable to improve the surface characteristics of the structure. As-wound or as-braided parts are typically characterized by uncontrolled surface topology which may be deleterious in service. For example, a surface of precise dimension may be required to enable joining the structure to an adjacent part or a surface of precise contour and smoothness may be required to achieve acceptable aerodynamic performance in aircraft applications.
In general, further consolidation of filament wound or braided structures cannot be achieved by conventional compression or autoclave molding. Attempts to apply pressure to the outside of a contoured as-wound or braided structure can result in collapse of the composite if the structure has a hollow core. If the core is filled, for example by a rigid mandrel, conventional compression or autoclave molding will still result in severe distortion of the composite as the fiber bundles buckle under the resultant compressive stresses.
A more preferred approach to consolidation of near net shape filament wound or braided structures involves applying pressure to the core of the structure and allowing the composite mass to expand against a rigid mold whose cavity follows the contours of the desired finished shape and has acceptable surface smoothness. In this way, the reinforcing fibers comprising the composite are subjected to a tensile rather than a compressive stress and no distortion occurs. Having been wound or braided over a mandrel which is removed, the as-wound or as-braided structure usually has a hollow core within which the consolidating pressure can be applied. Various approaches to the application of pressure in the core of structure have been reported. In one approach, an elastomeric bladder is installed in the core of the composite and the bladder is pressurized with a gas. In another approach a material of high coefficient of thermal expansion (CTE) is placed in the core. As the system is heated to a temperature at which the composite matrix becomes fluid, the high CTE material expands to a greater extent than the surrounding composite and, thus, forces the composite against the mold surface.
In both approaches to internal pressurization described above, a foreign material, comprising the bladder or high CTE material, is used. This places a significant limitation on the applicability of these processes since these materials must have acceptable performance at a very high temperature equal to that at which the composite matrix is fluid and has acceptably low viscosity. For many matrix materials, for example, polyetheretherketones, this process temperature can be 400.degree. C. In addition, this foreign material may infiltrate the composite structure with consequent deleterious effects. In medical implant systems, for example, the infiltration of a foreign material may compromise the otherwise biocompatible nature of the composite system.
It is an object of the present invention to provide a method of molding thermoplastic matrix, fiber reinforced composite structures to achieve a net shape geometry with enhanced consolidation and without temperature limitation and without the necessity to introduce a foreign material It is a further object of the present invention to provide a method of preparing composite orthopedic devices exhibiting minimal void formation and precise surface shape and smoothness. A feature of the process of the present invention is the adaptability of the molding process with other processing techniques. An advantage of the process described herein is the production of structures having tight tolerances to accommodate specific geometric envelopes. These and other objects, features and advantages will become apparent upon having reference to the following description of the invention.